Carrier telephone system



July 15, 1952 l. MOLNAR CARRIER TELEPHONE SYSTEM 11 Sheets-Sheet l Filed. July l0. 1947 Attorneys l1 Sheets-Sheet 2 l. MOLNAR CARRIER TELEPHONE SYSTEM .m mm mm mE o. $219@ 55m m M B @ON J EN i @om .wm .CCOO 8S Tvm. Nw N; ON @D 8N n w 52m Ivldm@ .o h.

Attorneys "July 15, 1952 l. MOLNAR CARRIER TELEPHONE SYSTEM Filed July 10, 1947 July 1 5, 1952, l. MoLNAR CARRIER TELEPHONE SYSTEM 11 sheets-sheet 4 Filed July l0. 1947 VAhorneys July 15, 1952 l. MOLNAR 2,603,717

CARRIER TELEPHONE SYSTEM Filed July 1o. 1947 11 sheds-sheet 5 ;#IIITRWB Attorneys July 15, 1952 1. MoLNAR CARRIER TELEPHONE SYSTEM Filed July 10, 1947 11 Sheets-Sheet 6 Attorneys l. MOLNAR CARRIER TELEPHONE SYSTEM July 15, 1952 Filed July l0, 1947 www o You Ahorneys www.

ll Sheets-Sheet 8 l. MOLNA CARRIER TELEPHONE SYSTEM July 15, 1952 Filed July 10. 1947 IMRE MOLNAR Aiornevs July 15, 1952 l. MOLNAR CARRIER TELEPHONE SYSTEM 1l Sheets-Sheet 10 Filed July l0. 1947 JNVENTOR. IMRE MOLNAR Attorneys July 15, 1952 I. MOLNAR 2,603,717

Y CARRIER TELEPHONE SYSTEM 4 Filed July 1o, 1947 I 11 -Sheets-Sheet 11 11 MESSAGE METER OR A) REGISTER II40 flug/t' RIISOLNILETER TO SUBSORIBER LIB' LINE lI INE S Rulo l TO "r T- Y FINDER Sw P /QOUT OEE I RHZO I C0 T c I l FROM CONNECTOR BANK y TO DISTRIBUTOR I I INE lCIRCUIT IIOO INVENTOR. -IMRE MOLNAR BY l v ATTYS.

Patented July 15, Y1952 2,603,717? CARRIER; TELEPHONE SYSTEM r yImre Molnar, Chicago, Ill., vassignor to Automatic J V .Electric Laboratories, Inc., Chicago, Ill., a c'orporation of Delaware g f .'Appiieationquiy i0, 1947, SerialaN, 7,760,136)

The present invention relates to telephone systems and more particularly to 'such systems of the intertoll dialing type-*employing carrier links.

It is a general object of'the invention to pro#- vide an intertoll dialing system involving a network ofautomati'c exchanges and ofces, provided with subscriber' s'ub'stations, paystations, manual ldialV -switchboards and automatic dial switchboards, lwherebyfth'e usual trunk and toll connections may .bese't-fup andf supervised'employing the carrierwlinks'.A f 1 1 v f Another objectl of the'inventior'i4 is to provide in a telephone .system lincluding 'aI remote -exchange of the automatictype, a tandem exchange of the automatic type,l vand a'mainoiiice provided with a switchboard, yan improved arrangement for settingfup connections involving the stations in the exchanges and the I.switchboard in the oce. :is 1:. y

Another object of the invention is to provide in a telephone-system :fof-the character described, an improved arrangement 'for setting up connections involving the' stations in the exchanges andthe switchboard in theV office .that comprise rst carrier links between the exchanges and second ycarrier linksffb'etween the tandem 'exchange and them'ain omce; whereby fulland complete' supervision'bf the: connections' involving the switchboard 'at the-'office' is vretained thereby." fm1 Y Another object ofv 'the invention is to provide in a telephone system of the character described, an improved 'arrangement .for permittingl free calls from the stations in the different exchanges to the switchboard in' the 'office VandiforV assess- .Y 35 claims. (crm-27).-

ing charges in connection with calls between the stationsk in the differentiexchanges; 'w

Another object vofthe invention vis `to provide in a telephone systemof the character described, an improved arrangement for blocking calls from the stations'in'the .remote exchange to 'the stations in the tandem exchange when the outgoing route selected is that normallyxempl'o'yed to extend the calls -from vthef'stationsv in the remote exchange to the `switchboardin the main oice via the' automatic .'switchingvapparatus in the tandem exchange.' 1;

Another "object of the invention is to provide in a telephone system ofthe character described, an improved arrangement for-permitting the'free calls mentioned and for assessing charges in connection with the calls 'mentioned'an'd for blocking the vcalls noted' 'regardless of whether the stations in the'remoteex'change originatingv the calls' are of the subscriber substation type orA of the paystation typejm'. 1 -f A further object'fof' 'thev invention' isto provide in atelephone" system including n rst and second trunk'circuits interconnected 'by' a'trunk, an improvedarrangement wherein thelrst trunk circuit includes a source of ordinary battery'potential andiv a source of booster battery potential and thef'sec'ond ltrunk'circuit includes a source of return potential Aand. 'asymmetrical' current conducting equipment, whereby a 'connection may be extended from a rst line incoming to the rst trunk circuit to'a second line outgoing from the second trunk circuit utilizing the items named in the two trunk .circuits and supervisory signals 'may'be 'transmitted-in' either 'direction between thelinesfas-.required f i .i

' A .still lfurther' object of'the 'in'ventionl is'to provide a-te1ephone= system including rst and second `trunk` `circuits 'interconnected 'Sby a trunk; an improved'arrangement' where'inlthe first vtrunk 1 circuit -i'ncludes "af source of ordinary batteryk potentialA and 'arsorce 'of booster battery potential `and 'the second 'trunk `.circuit includes a vsource of* return potential 'and as'ym'metrical current kconductingequipment; whereby 'a connection may be extendedfrom a vsecond line incoming 'toth'e second trunk' circuit to a first line outgoing .from the "rst Ltrunk' circuit 'utilizing 'the items'namedin the two trunk" circuitsand supervisory signals` mayber transmitted in either directio'nbetween the .lines as required. 'i

Y Further features of rrthe .inventionp'ertain 'to the particular arrangement-'of thev 'circuit elements cfithe system, 'whereby the .above-outlined andaddtio'nal .operatingffeatures thereof are attained; ..".The invention, both as to its .organization and method' of operation,` together' withiurther objects and ladvantages thereof; will best be understood by reference tothe following specication taken in, connection withv the accompany'- ing.drawings;iirwhichzFigure 1 is a diagrammatic illustration of fan automatic telephone sys tem embodying-the^presen`tl invention and comprising zat'rernote` exchange*I.0f,"a'y tandem exchange 20, a way"- station "30j "a main .oiic'e"40 'and' a .distant 'oice :50;I andi Figsf2 'to"1'0`,"in'- clu'sive,1taken together, illustratethe detailsio'f the apparatus incorporated' in the` automatic tele'- ph'one system illustrated in' lr-and' embody#- ing the features of the' invention as briey outlined'ab'ove; Figs. 2 to 10, inclusive, should4 be arranged from'A left tofright in' end 'l-toV-'en'd relation 'inordrto form` ja" unified diagram@ "More/'particularlyFigs, 2 and 3 illustrate 11agrarr'irnaticall'y the! automatic switching'y appa:

ratusand' one'of th''pieces of carrier equipment disposed in the remote exchange ,I as well as the Y details of one of the trunk circuits disposed there- Y in; Figs. 4, 5 and 6 illustrate diagrammatically station 30,'as well as the details of one of the f trunk circuits and one of the alarmlsenders dis:- posed thereat; Fig. 7 illustrates dia'grarrimaticalli vidual" thereto. Also thevtrunk circuit 20D cornprises two additional trunks 220 and 225, both Y accessible to the selectors of the different groups one of the pieces of carrieri'equipmentrlisposecL, at the main oflice 40, as well asthe'fdetai-ls of.'

one of the trunkscircuits, one of the alarmreceivers, and v,one of the cord-.zcircuitsidisposed1 therein; Fig.- 8 Yillustrates diagrammatically the.

automaticswtching apparatus and onecfr-"the:-y

pieces ofV carrier equipment disposed in the main oliicefmzas-wel-lras -.the';detailsz.of;a dierentone ot..fthe-.=;trunlfs circuitsdisposed: itl'lerein; ,Figsvsl 9 and 10.illustrate-.diagrarnmaticallgn the; automatic switching; apparatus; andvzoneaofzthespieces. of; car-v rier equipmentsdisposedffing;thegdistant; oflcef 0 disposed therein; andvFignl-l .illustratesf-thedetails-of a conventional line. circuiti,oiztheareversebatteryicontrolledchargefassessinggtyp The @automatic-telephone :systemf Referr'ingf'now-inorefparucmariytaria-1, the automatic `telephone systenrthere illustrated com--` prises the remote .exchanger- I0:se1'ving;a:nu1nber scriberrfline 2 I -I f extendingjheretcifand zhavingga line;circuitv 2 I3 individually -associated-itherewith and A'the paystationaT I 2 5 isiprovide'd' 'withia'sline' 2 12:; extending jgtheretd'gandfl'lavrigfa 1 line :circuit 2 t4 1 individually associatedztherewitlni. Also'gthe remote; exchanger.Ill-comprises ea' pluralitysz-of r groups .'offnderfselecton'links; the iir'stfgroupnf Y cluding: the; iinder-'selectorj'link-Z 0 3 and 'provided withsa idis'tribiutor 204; vas ;vvell asf. agpluralityuof groups ,of flocalsconnectdrsgthezrstgroupiorlocail connectorsz'gincludingthe. clonar-connectors 217.5,

Preferably, vthessubscribierliner a2 I;`If extending! to the subscriber..:substation:Tlrlrand:theQ-line `212 extending o toilthe ipaystationFI 2 tare? arrangedin the flrstgroupofiines and areaccordingly :acces:-

indereselectorlinks12H3,A etc.; andpta .thelirst group. of: local lconnectors: 2Il5;,:;:etrr.:` Frtl'lercthe remote exchange Illicompriseszagroup ofzzincome -ingselectors 'including the incoming se1e'ctorlf206;

havingziaccessfa tori-the .differentrgroupsxo I local connectors: theA local'connector-2iinxtlre rst group #being-f` accessibleto1tlie1 incomingisele'ctor 206A via the v'trunk-2li.'If extending 'atheretcix Thev remotewexchange- .rI 0i aandthe: itandemfexe sible both to the' finders of the rstgrouplof of iinder-selector links.V Finally, the selectors of the different groups of iinder-selector links in the remote exchange I0 also have access to the different'rg-roupsi otlocaliconnectors therein. Specifically, "the selector o-th'e finder-selector link 203 has access to the local connector 205 via -the trunk 201 extending thereto.

The-tandem exchange 20 serves a number of ordinary 'subscriber substations, including the subscribenisubstationflZ I, and a number of pay- 'stationspincludingthe paystation T22. The sub- 'Yscriberc-substation'T2I is provided with a subscribenlineAZI; extending thereto and having a line circuit 423 individually associated therewith; and-rthepaystation T22 is provided with a line 422 extending thereto and having a line circuit 424 individually? associatedtherewith;A- Alsd the tandem-exchange Zfcomprises if. affpluralityftof groups fofY nderg-selectoivlinks; .fthe rst vgroup in-` cluding thefnderfselectordink 426e and provided ywithA a distributorrid 25; asawell as ;a ipluralityV of groups of? rlo'cal; connectors, @thegirst-v groupyfof local: :connectors ..in'cludingfz the .f1 local. connector 4211 Preferably-.the subscriberlinelZI extend,- ing tothe @subscriber substatonifIZ Iv and the lline AZZeXtendingA-ftotthe paystation T22 andarranged in the first group of linesfand-fare accordingly accessible".i bothgto fthe finders of: ithe iirst 'group of;ndereselectorlinks:1426;.-etcs, anolltozthe frst groupgci" localvccnnectorsLZl;:etc Further,v the tandem:M exchangeuZU rcomprises: a1.:f1rstz group offlincoming selectors; includingv eitlie; incoming selector 428-, Khavi'ngfaccessz*,toethe: diierent.v groups of ilocal' connectors; and f'agseccndfgroup ict; in' coming selectors, including the incoming selector 4391;: having? accessz-zto th'e :diierent groupsof local;.connectors;r` the-locahconnector' 42T inthe Yfirst; groupxzbeingn accessible; f toi; thef lincoming selectors: Y:4 2t;r et'c.-;.1anrl:'43,4i,1etc:,1 vvia the ftrunk 42'8'f-,fextending gthretos lThescarrie'r-f equipment 49.4; inzzthe; tandem-,f exchange 2011is connected to a :.trunkxcircuit v4 lzrindvidual. thereto; which trunklcircuitx' iscccnnected 'Lb'y a: trunk: 435 to.;.;the: incominglselector: 4291' individualv thereto; Alsofithe .'trunkflcircuit' '4Ilcomprises ani. additional trunk 430 accessibleto the Vselectorsof the `fdiierent groups: of 1l1"ld -5r'selector links `and `6 8,5." terminatings-'fin the '.trunk' circuitsv .5ml and change n-2Ilrare interconnected-.by a plralittefof channels of`y carrier-equipment thei'rstrchannel illustrated fincludin'gsthe :line 6U extendingfbbexchangesZUi'and-respectivelyterminating irl-:the

carriertA equipment; ;394-Ipro.vidediinthe :remote f exchange I (I vand in :thefcarrier equipment; :4 34 .providedsinrtheftandem exchange 2li: :Thezcarrier equipment394 is connected-toca trunk cirouit1200 individual thereto vvvv -vJgu--ovidedfinfthe remote Lex- .changai 0;; which ftrunk-,fcircuitf2 olifisnconnected ,I

by@ .trunkmfte thepmcemmg summercamp;-

IiIlIlll respectively: provided. in thetandem exchange `2 Ilv andi'at the wayrstationill. The trunk'f circuit lofis iccnnectedbyfaztrunkA 52!4 tothe incoming seleetor1434 :individual .theretoz VAls'o-zthe trunk circuit 500i' comprises: ,twof additional trunks 510 and@ 5 I5 :respectively: :accessible ato. the selectors ofthe diierentf'groupsof iinderselector links and to the incoming selectors 429,-fetcif fl'inally';` the selectors :ofthe different :groups :ofirder-selector linksfinzthe-tandemexchangeZDfalso'xhave'access toith-e differentgroups ofilocalconnectors therein.; Specically; the -selector;"ofthe rfinder-selector: link. 426 has-:access:torlthe localV connector 421 via' the-trunk' V4 28;'extendingxthereto.

The lwzm'statiorr?.Il'andzthemain Loiice 4 Ii-Iare inter-.connected byr-afvplurality of? channelsl of carrier equipment; the rstchanne1};illustrated 'tandem exchange 20 andthe way station'30, as

previously noted. Also Va plurality ofv alarm senders 680, etc. are provided at the Way station 30 and respectively connected to the carrier equipment 694, etc., thereat.

The carrier equipment 194v provided in the .mainoice 40 is connected to a trunk circuit 100 individual thereto; which trunk circuit 100 terminates in a jack J 130 appearing before an operator switchboard 165 in the main ofIice 40; which operator switchboard 165 comprises a plurality of cord circuits, including the cord circuit 166. Also a plurality of Valarm receivers 110, etc... are provided in the main oiiice 40 and respectively connected to the carrier equipment 194, etc.

The vmain oftice y40 serves a number of ordinary subscriber substations including the. subscriber substation T4|. The subscriber substation II4I is provided with a subscriber line 84| extending thereto and having a line circuit 842 individually associated therewith. Also the main oce 40 comprises a, local switch train 844 provided with a distributor 843. The subscriber line 64| extending to the subscriber vsubstation T4| has access to and is accessible to the local switch train 844. Also the main oice 40 comprises a group of C. L. R. trunks'including lthe C. L. R. trunk 846. The C. L.v R.. trunk 846 is accessible to the local switch train 844 via atrunk 845 extending thereto and is alsoprovidedl with a trunk 841 extending therefrom which, terminates in a jack J848 appearing before the operator switchboard 165. Moreover the main office 40 comprises a toll switch train 805 vhaving access to the lines 84|, etc., respectively extending to the subscriber substations T4I, etc.; vwhich toll switch train 805 has a trunk 806 extending thereto and terminating in a jack J 801 appearing before the operator switchboard 165. Further, the main oiiice 4|) comprises a group of incoming selectors. including the incoming selector 8 I 2, having access to the toll switch train 805 via the trunk 806 extending thereto. Also the main oillce 40 comprises a group of toll trunks'inward 809, etc.

The toll trunk inward 809 is accessible to the incoming selector 8|2via the trunk 808 extending thereto and is provided with a trunk 8|0 extending therefrom and terminating in a jack J8|| appearing before the operator switchboard 165.

The main oice 40 and the distant oii'ice 50 are interconnected by a plurality of channels of` carrier equipment, the rst channel illustrated including the line 80 extending between the main oiiice 40 and the distant office 50 and respectively terminating in the carrier equipment 894 providedv in the main office 40 and in the carrier equipment 994 provided in the distant office 50.

VThe carrier equipment 8.94 is connected to a trunk circuit 800 vindividual thereto inthe mainoilice 40; which trunk circuit'r800 is connected by a trunk 815 to the incoming selector 8|2 individualthereto. Also the trunk circuit 800 terminates in a jack J 8|5 appearing before the operator switchboard 165.l

The distant oiiice 50 serves av number of ordinary subscriber substations, including'the subls'criber substation T5|,.ff1he subscriber substation T5| is provided with a' subscriber line 105| extending thereto land having a line circuit |052 individually associated therewith. Also the distant oiice 50 comprises a local switch train |054 provided with a distributor |053. vThe subscriber line |05| extending tothe subscriber substation AT5| has access to Yandis accessibleto the local switch train |054. Also the distant office 50 'comprises aVStrowger:- dial-.toll'board |060 pro- 'vided with apluralitylof toll operator positions,

one of which is indicated at I06l. Each toll operatorposition is providediwith agroup of finderselector links individual(theretoythe. group of finder-selector links .individualto the toll operator position |06| 4including the finder-selector link |062. Furthenthe distant officeV 50 comprises a group of C.v L. R. 'trunks v, including the C. L. .R. trunk |056; tl1ey C. L. R; .trunk |056 is accessiblev to the'jlocal switch train |054 via a trunk |055 extendingthereto; which C. L. R. trunk circuit r|056 is also provided with a trunk |051 extending therefrom vthat is `accessible lto the iinders of the'flnd'er-selectorlinks associated with the various toll .operator positions atthe Strowger dial toll board |060. Moreover,` the distant office 50 .comprises -a toll switchtrain 038 having access tothe lines|05| ,l etc., respectively extending to the subscriber substations T5|, etc.; which -toll switchv train |038 has af. trunk .|031 v extending thereto that' is accessible to'the selectors of. the vfinder-selector links associated t withl thel various. toll .operatorv positions -atl-the Strowger dial vtoll board `|060.` '-.Furtherg the distant oflice 50 comprises agroup of incoming selectors, including thevincoming selector |036, having access tothetollfswitch train=l038 via thetrunk .|031 extending thereto. Alsothe distant `oce 450 Vcomprises a group of toll trunks inward |066, etc. 'Thetolll trunks inward |066 are accessible to the `incoming selectorl |036 via a trunk |065 extendingthereto and is provided with atrunk |061 extending therefromand accessible to-.the nders of thefinder-selector links associatedwith the various ltoll operator positions at the-Strowger dial toll board|060.

The carrier equipment 994 provided in the distant ofce 50 is connected to' the trunkcircuit 900 Vindividualthereto;` which trunk circuit 900 1s connected via' a 'trunk |030 to the incoming selector |036 individual thereto. Also the trunk circuit 900 is provided with a trunk-|040 incoming-thereto that is accessible Vto the selectors of the finder-selector links` yassociated with the various toll operator positions at the-'Strowger dial-toll board'|060.-` f' y Y incorporated in the automatic The apparatus telephone systeml Preferably, the carrier systemfinterconnecting `the remote exchange |0v and the tandem exchange- 20, the carrier system interconnecting the rway station 30 andthe main oice 40 and the carrier system interconnecting the main office 40and the distant office 50 are identical and each .comprises atypeSZLenkurtunit of the characvter disclosed in the copending application of Imre Molnar, Serial No. 728,422, filed February ,'-14,"194'1,lnow Patentl No.12,559,165, granted July -3, 1951. terconnecting the` variouszexchanges, oiiices and "Alternatively, thejcarrier equipment inway stations mentioned maybe ofthe radio-link Y(13.1 to. 15.7 kc.) .for communication purposes, receive a carrier -frequencyof 24.55 kc. for signalling-purposes, 4and vreceive "the lower side vband of the carrier frequency24-55kc, (21.7 to 24.3 kc.) forfcommunication purposes..v Similarly, the carrier equipment, '394,1694 .-a1f1d5894 respectivelyV disposed. at the -west ltermini of. the lines 60,10 and 00, and respectivelycomprising portions of the first channels inthe three carrier current systems` mentioned, .transmitat a carrier frequency of 524.55 kc. Vfor signallinglpurposes, transmit the lower side band ,of the carrier frequency 24.55 kc.

(21.7 to 24.3 ,kc.), forcommunication purposes, receive a carrier 'frequency of 15;95. kc. for `sig-- nalling purposes, and receivegthe lower side band oi the ,carrierfrequehcy 15.95 kc. (13.11to 15.7 kc.) for communicationpurposes. Y

Also, each .carrier equipment 394, etc., com prises lan outgoing signal transmitting channel including a rst. carrier frequency oscillator and a transmitting amplifier providedwith a plate Acircuit controlledby. an associated signal-out relay; as wellas an outgoing communication channel including a, balanced modulator connected to the first carrier frequency v.oscillator and an associated transmitting. bandi pass filter. A lso, each carrier equipment mentioned comprises an incoming'signal channel including a rectifier and a signal-in `relay;; as Vwell asan incoming communication channel including a balanced demodulator-connected to asecond carrier frequency y oscillator and an 'associated receiving band pass filter. Also each 'carrier equipment mentioned comprises a transmitting direction filter connected to the associatedoutgoing signal transmitting 'channelvan'd to ther associated outgoing communication channely-as well as a receiving direction ltercon'nectedt'o theA associated incoming signal vchannel-and to the associated incoming communication channel. Further, each carrier equipment mentioned-comprises a high pass line: filter connected'between the associated line terminusand the transmitting directonfilter and the receivingdirection lter. ll'inally, each line terminus i'sjconnected by a low: pass filter to other `voice frequencyY equipment. This conventon'alequipment described above and disclosed vin the previously mentioned copending Molnar application, permits utilization of the lines 60, 10 and 80 in other voice frequency equipnment ina manner well understood.

As shown in "Figf2, preferably each of the finder-selector links 203, etc., each of the local connectors '205, etc., and each of the incoming selectors :206, etc., are oflthe conventional Strowger types; while'thedistributor 204 and the line Acircuits 2I`3; 2I4, etc., 'are of any suitable types.

to; and each yofthepaystations'TI 2, etc., is ofthe ycharacter of .that'disclos'e'd Vin Fig. 3 of rUnited States Patent No.' 2,137,409, vgranted November 22,. 1938, to Talbot G. vrMartin and Clarence'E Lomax. j-Specicallyfa paystation of this ftype comprises', among other Lv-elerrients, aV transmitter `bridgedby an asymmetrical current conducting A:8 device gin the;y form of .-.a copper*l oxide rectifier; whereby a person at thepaystation may .control the extension cfa connectiontherefrom and converse withl an operator without depositing a coin thereat. Also, the-personatthe paystation can normally extend a connection therefrom to a subscriber -substatiombut cannotr talk .to the subscriber thereat until a coinhasvbeen deposited` at the paystatiomas a rconsequence of the blocking actionrof the copper-oxide rectifierv with, respect to the transmitter when the polarity of theline is reversed in alocal connection.

Preferably, each ofthe trunkY circuits in the remote exchange I0isi'denticalto the trunk circuit 200 that comprises, as shownin Figs. 2 and l3, a relay group including. a timer relayrR235, a reverse batteryfrelayR240, .a series relay R250, a switch relay R260, asupervisory relay R210, a timer relay R230, an answer relay R3 I0, a dial relay;R32`0, a line relay R330, ahold relay R340, a signalfin relay R350, two digit .relays R360 and R310, a hold relay R380 ,and atone relay R330. AlsoQthe trunk circuit `200v .comprises an impedance 251 anda busytone transformer Bilt.y At this point it'isnotedv that the trunk circuit 200 is connectedto the carrier y'equipment 304by a pair ofy line conductors .C35i and C302,.a signal-out conductor C303and a signal-inrconductor C304;

Vand that'the apparatus .provided in the carrier equipment 394, as previously explai-ned, com prises a signal-in relay. .R395 anda rsignal-out relay R391.. y

-lnthe tandem `exchange 20, as shown. in Fig. 4, Ieach of the iinderfselectonlinlzs 425, etc., each of the local connectors4l21, etc., and each of the incoming selectors 425, etc.,v and 434, etc., Yare fof the conventional -Strowger types; whilel the dis'- Ytributor 425 and 'the line .circuits 423, 424, etc.,

are of any suitable Vtypes. zFinally, each of the subscriber substations T2 I, etc.,is provided4 with conventional substation apparatus; each of the subscriber lines 42 I, etc.,- is provided with a meter, notshown, individual fthereto.;v and each of the paystations T22, etc., is of the .character vof that disclosed in Eig..3 of the previously mentioned Martin et al. patent.A

In the Ytandem exchange l2 0, preferably each of the trunk circuits inthe-group, including the trunk circuit .4.00, -isidentical thereto; which trunk circuit 400 comprises, as shown irl-Fig. 4, a relay group including a supervisory relay R440, an answer relayR445 a signal-in relay R450, a dial relay'R455', a switch relay R480a line relay R410, a hold relayR480, acontrolrelay R490, and a hold relay R400, as .well asanv impedance 45B.

At. this ypoint it .isnotedthatfthe trunk circuit 400Y isconnectedtothe carrier. equipment 494V by a pair of line'conductors 040| and C402, a signalout'conductor C403 andasignal-in conductor C404; and that theapparatusprovided in the carrier equipment. 494,.as previously explained, comprises a signal-,in'relayR495 and a signal-out relay R491.

Also, in the tandem. exchange 20, preferably,

.each of the trunk circuits in the group, including the trunkcircuit ,500,.is identical theretoywhich trunk circuit .500 comprises, .as shown in Figs. 5 and 6, a relay groupincluding a reverse ,battery relay R530, `two timer relays R540 and R550, supervisory relay R560, a tone relay R555, a hold relay R510, a supervisory relay R500, a seize relay R505, acontrol relay R590, ani-answer relay R595, a line relay R610, a pulse relay R620, `a switch relay R630, and a dial. relayJR640. Also," the trunk 'circuit'1500 comprisesfan ordinary battery comprises two asymmetrical current conducting devices 668 and 6 61, lin theform of copper oxide rectifiers, and a repeating -coilv-RC615. At this point it is noted'that the trunk circuit 600 is connectcdto the carrierequipment694 by a pair of line conductorsC60| and C602, a `signal-out conductor vC603 anda signal-in conductor C604; and that the apparatusprovided in the carrier equipment 694,. as previously explained, comprises a signal-in relay R695 and a signalout relay R691.

Preferably, in the way station 30;,each of the alarm senders is identical to the alarm sender 680 which comprises, as shown in Fig. 6, a motor 68| operative from a commercial source of single phaseA.- C. power and operatively associated with a set of switch springs S682 and with a gearbox 683. The gearbox 683g is adapted to time operationf of the lmotor' 68 andat the expiration Aof ten minutes to open the associated contacts S684 and toclose the associated contacts S685.v Further, the .alarm sender `680 comprises a start relay R686.

In the main oflice 4 0, preferably each of the trunk circuits in the group, including the trunk circuit 100, is identicalthereto; which trunk circuit 100, as shown in Fig. 7, terminates in the jack J130befo're thev operator switchboard 165, as previously noted, and'comprises a relay group including-asignalin relay- 12.105, a jack relay R1|0, anda sleeve relay 112,120;V Also, the trunk circuit.100comprisesanbusy lamp .L132 and a line lamp. L13; disposed at theoperator switchboard 165,.; At this point it is'noted that the trunk circuit 100 isconnect ed tothe carrier equipment 194 by apair or line conductors C| and C102, a signal-out conductor-C103 anda signal-inconductor C104; and that Vthe apparatus provided in thev carrier 'equipment 194,A as previously explained, comprises asignal-in relay R195 and a signal-out relay R191.;

Preferably, each ofgthe cord c ircuits disposed at the operator switchboard165 is identical to the cord 'circuit 166 which, as shown'in Fig. '7, terminates into. front and rear plugsP148 and P158, respectively, and comprises a dial D160, a dial key K131, a ring key K136, a talk front key X134 and a talkrear'key K135. Also, vthecorcl circuit 1.66 lcomprises `a front supervisory lamp L141, a rear supervisory lamp L151 andaJ relaygroup including a'dialfront relay. RI,4.0,; asupervisory vfront re1ayRJ45, a dial rear relay.R150.and a supervisory rear relay R155. Finally, it is noted that the cord circuit 1'66 is associated with a headset 161 disposed at the operator switchboard 165 and with aninterrupted generator 138.

Preferably, in the main office 40. each of the alarm receivers is identical to the-alarm' receiver 110 which comprises, as shown in Fig. '1, a motor 113 operative from a commercial source of single phase A. C. power Aand operatively associated with two sets of switch springsr S111 and S118 and with a gearbox 114. The l gearbox 114 is adapted to time operation of the motor 113 and at the expiration of 91/2v minutes to close the asso-l ciated contacts S115 and at the'expiration of 10i/ minutes to close the associated contacts S116. Further, the alarm receiver .110 comprises a relaygroup including an alarm relay R180, a start relay R,185,ja hold relay R1904anda1timer relay 110i R100. Finally, the alarm'sende'ar100 comprises an alarm lamp L11 a delayed alarm DA112 and a reset key K113.

In the main ofce .40. preferably 'each' of the trunk circuits in the group, including the Y.trunk circuit 800, is identical thereto; which trunk circuit 800 terminates inthe jack J8 5 before the operator switchboard 165, as previously noted and comprises a relay group including a sleeve'relay R820, a signal-out" relay R830, 'a jack relay R835,

a hold relay R850, a dial 'relay R855,` lasignal-inv relay R860, two' re-ring relays R865l and R810,`

a switch relay R880 andan answer relay R890. Also, the trunk circuit 800 comprises arepeating coil RC815, as well asa busy lamp' Llldisposed at the operator switchboard 165. Atthis point it is noted that the trunk circuit 800` is lconnected to the carrier equipment 894 by" a' pair of V'line conductors C80| and C802, 'a'signal-out conductor C803 and a signal-in conductor.C804;l-and that the apparatus provided in the carrier .equipment 894, as previously explained.' comprises a signal-in relay R895 and a vsignal-outrelay R891.

In themain office 40, as shown in Fig. 8, preferably the local switch train 844, the toll Switch train 805 and the incoming selectors 8 |2,.tc., are of the conventionalStrowger types; while thedistributor 843, the linev circuits1842, etc., the C. L. R. trunks 846, etc., and the vtoll trunksinward 809,. etc., are of anysuitable types'. Finally, each of thesubscriber substations T4I, etc., is 'provided with conventional substation apparatus.v

' In the distant oflice 50;"preferably each of the trunk circuits is identical to thetrunk circuit 900 which comprises, asshown in'Figs. 9 and 10, a

relay group including a signal-'in relay R920, a

hold relay'R930, a .dial relay R940, a trouble relay R950, a seize relay R955, a switch relay R960, a re-ring relay vR910'a'nd an end-of-'dial'relay R980, an answer relay R915, a line relay R| 0|0, a trouble relay R|020 and a re'ring relay vR|||25. Also, the trunk circuit 900 comprises a repeating coil RC9|0. At this point,'it is noted that the trunk circuit 900 is connected to the vcarrier equipment 994 by a pair of line conductors C 90| and C902. a signal-out conductor C903 and asignal-in conductor C904; and that'the apparatus provided in the carrier equipment V994, as pree viouslyexplained, comprises ia signal-in relay R995 and a signal-out relay R991.

In the distant office 50, as shown in Fig. 10, preferably the local switch" train |054, the toll switchtrain |038,l the incoming selectors '1036, etc., and the finderselector links '|062, etc.,' are of the conventional Strowger types; while the. distributor |053, the linelcircuits |052, etc.. the

C. L. R. trunks |056, etc., and the toll trunks inward |06'6,et'c;," are of any suitable types. nFurther'the Strowger. dialv toll board |060r is ofthe connectionand arrangement of that .disclosed in U, s. Patent No.2,353,212 gra nped July 11,1944,

to Harvey W. Balzer. Finally, each of the subscriber substations T5|, etc., is providedwith conventional substation apparatus..I Each of the line circuits 2|3, etc., 423, etc., 842, etc., and |052, etc., maybe of the general character of the line circuit |00 shown in Fig. 11.

A better understandingof the connection `and arrangement of the apparatus incorporated in the automatic telephone system illustrated in Fig. 1 and brieflyA described abovegwill b e facilitated from a consideration of the extension of connectionsv involving` the differ'entfexchanges and oiiices as willappear hereinafter;

erano v Local calls-in the remote exchanged@ tirs assume tha-ta @au initiated at :1i-subscriber substation such, for example, as" the subscriberv substation` TI IA in theV remote `exchange 'I 0 Vis to be-extended to'apaystationsuch, for 'ex ample,'as

the paystation TI2 'therein' lWhen the callv is initiated at the callingjsubscriber substation TI I, the lune circuit z rafasis'ociated with the subscriber linejZI I', operates tofmarkithe subscriber line 2I I asbusj :to j the local'j connectors 205, etc., having aisc'e'ssV thereto; "andi toQiniti'ate operation of the distributor'dZflff The jdistributor 202i assigns an y vdirectory'l'rn'inibertof the called 'paystation TI2,

which `be" r112. The Selector of the lldelslectoi'flink `2037re`sp1`1d`s t0 the first digit 1 t0 'selectftl'iev rst groupj'o'f local connectors, including the "local connector 205. and then an idle V loc'zalc'o'n'nectcr ltherein. For' example, Y'the selector of 'theindern-selector link -'22.03 may select th'eitruk .201 e:tieneing`y to the "particular local the line 2I2 'extendingfthereto asbusy returning y busy tone current over the trunk`,.201, the nderselector link 203 and the subscriber line 2l I to the 'callingsutscribersubstaucn' Tl I inV order to indicate to the subscriber therea-t that the Vdesired connection' may not `be had at this time due to the busy conditioner the called paystation TI2.l The y' subscriber` at the calling subscriber substation rlII then proceeds 'tdreleasethe apparatus in a manner lmore fullyexplained hereinafter. Y,

Now assuming that the called paystation TI 2Vis idle atthistime, the local connector 205 operates to .project Tmginefurrent over the une .212 to the called paystation 'I I2 and to return ringback tone current over .the trunk 201 and, conorder to cause: the local: connector 205 and the finder-selector link 203 to be released. "When the local connector 205 is thus yreleased,the line 2I2 extending to the paystationTlZ is again marked asidle by the associated line circuit 2M vto the local connectors 205, etc.,A hat/ing access thereto. Similarly, when the: nder-selector link 203 is thus released, the line `2| [extending to the subscriber substationTI I is markednas idle bythe associated line circuit 2I3 'to the local connectors 205', etc.; having access thereto. Also, when'the nder-j-selector link i203' is 'thus released, the trunk" 201v extending to the localconnector -205 is marked as Aidle to the nder-'selectorlink's203, etc., hav-` ing' access thereto. -At-thistime the established Y connection betweenV the 'cal-ling subscriber sub- 'station .Tn andthecailedpaystation TI2 is re-` leased and all of theappai-fatusinvolved ltherein is completely released and'aifailable- .for furtheruse.

In view of the foregoinggit'will be understood thatf'a connectionrnaybefextended from a calling subscriber substation A-II Ietc., in the remote exchange I to another called subscriber substa-y tion therei-n^`in anidentical manner.' Also, it will be understood that a'connectionfroma 'cal-1- ingV paystation yTI 2;' etc., in 'the remote exchange I0 may be extended'to'a calledsubsc'riber substation TI I, etc. vor to anothercalled paystation therein in asubstanti'allyi identical manner. In this conjunction, itis no'tedincident to extend*- ing a connection -fromthe callingr paystation TI2 in the remote exch'ang'ell to a called subscriber substation or to anothen'called paysta'tion therein, that when the calli isanswered,v the local connector 205 operates yto reverse the polarity of the trunk' 201 fand "over" tlriefnder-selector link employed and fthe' line 32 I'2j`1t'o the' calling paystation TI-2'. vThis'reversal'of polarity to the calling paystation TI2- brings "about vblocking of vthev transmitter at the calling-paystation'TI2 bythe associated copperoxide rectifier unless the person at `the pays't'atio'n vTI2 has deposited a coin at this time. More particularly, in the event the person `at the calling' paystation TI2 has not deposited a coin at this time,` he may hear the person at the called subscrib'ersubstation or'atthe called paystation in thel'rmote exchange I0, but heI may not talk to .thelperson mentioned by virtue of Vthe -circumstancethat the copper oxide rectifier blocks the" associated transmitter as a consequence of the lreversal of=po1`arity to the calling paystation TI2. v Hcweven'when the'perseduently, to the" calling. subscriber substation TI I inorder to' indicaterto'the subscriber thereat that the called paystation TI2 is being rung.

When a person at the called paystation T I2 answers the call, the local connector 205 'operates iii-order to reverse the polarityof the trunk 201; whereby the Ender-'selector link 203 operates the meter, not shown-associated with the subscriber line 2II in order to meter the call -in a conventional mannen YAt this time, an established con'- nection vis completed between the calling subscriber substation TII and the called paystation TI2 and conversation .therebetween then proceeds in the usual manner. Y

At the ,conclusion o f the established connection, when the person atthe paystation TI2 disconnects, theclocal connector 205 again voperates to reverse the polarity of the trunk 201; how-` ever, without eiect at this time. Subsequently, when thesubscriber at the calling 'subscriber substation TII disconnects, the loop circuit extending to the local connector 2051s interrupted'in son at the calling 'paystatio'n TI2 depositsa coin,V

the' copper 'oxide `rectifier-is removed' from block.- ing the transmitter thereat; 'whereby utheconf nection is completed'- in the k'manner previously explainedg' v Y A call" from a'subsczz'ber substation or pdg/'station in the remote exchanged() 'to a' subscriber Vsulxstcntio'n, or pag/station` in' the tandem 'eccchange-'20 First assume that a call initiated-Lata subscriber substation such, for example, as the subscriber substation TI I -in the remote exchange I0 is to beextended toa subscriber substation such, for.' example, as the subscriber substation T2I in the tandem exchange 20. When the call is -initiated at the calling-` subscriber substation TII, the line circuit2l3fas'sociated with the subscriber line 2II operates to mark the subscriber line 2II as busy to the vlocal connectors 205, etc., hayingaccesswthereto, and Vto initiate operatiori'of th distributor 204. The'd'stribiltor 13 204 a assigns an ...idle one. of the finderselector linksV such, for. example, as the nder-selector 'link 203 inthexrst vgroup for useatthis time; Ywhereby the finder lof the finder-selector link 203 operates to seizethe subscriber linef2| The subscriber vat the calling subscriber' substationrTl in the. remote exchange I then dials a prefix digit such, for example, as the prex digit 9, indicating that the call is to be extended to the tandem .exchange 20,. followed by the .directory number of the Acalled subscriber sub- `station T21 .inthe tandem exchange20, which number may be. 121. Thus the subscriber at -the calling subscriber substationl TII first dials .the control conductor C223 thereof. vMore particularly, battery potential is normally applied by way -of the impedance 251, the contacts 266 and 383 to the` control conductor C223 of the trunk 225.1

'.Also,lwhen the" selector of the finder-selector Alinkf'203 seizesrthevtrun'k 225, a loop circuit is completedY for f energizing the upper and lower windingsof the line relay R330 in the trunk circuit 200. This y'loop circuit extends from ground by way of ythe contacts 362, the lower winding of the vline relay R330 and the contacts .'244 to the line conductor C221 of the trunk 225;

and frombattery potential by way of the upper winding of theline relay R330 andthe contacts .242 to the line:c0nductor C226 of the trunk 225.

Whenthis loopcrcuit is thus completed, the line .relay R330. operates vto complete, at the contacts: 33|, anvobvious circuit for operating Athehold relay R380.v AUpon operating the 'hold relay AR380 .interrupts, atl thecontacts 383, a point in the preyiously traced path for applying battery potential tothe control conductor C223 of the trunk v225; and completes, at the contacts 384, apath includingthe upper winding of the tone relay R330 for applyingground potential to the control conductor C223 of the trunk 225. When battery potential is. thus removed from the control conductor'C223 and groundy potential *isi applied thereto; the trunk 225 and, consequently, -the-trunkcircuitk 200 is marked as busy to the selectors of the other finder-selector links `.inthe'platetcircuit ofthe associated transmission .ampliiier;wherebyl.;the v24.5l kc. .carrier signal from 'theassociated carrerirequency oscillator `is transmitted :fori'signalling purposes over Vthe associated line iextendingbetween the remote exchange l0 and thetandem exchange 20.

When the 24:55'kc. carrier signal is received in the carrier equipment '494 disposed in the -.tandem exchange `20, thecarrier equipment 494 ated` signal-in` relay yR495 therein; V,whereby the havingv accessA thereto.: VAlso the upper winding of the tone.' relay R390 is-energized causing this relaytofoperate andcomplete, at the contacts 39|, an obviouspath for applying ground poten- -tial to the tonefstartI conductor C349,thereby .to initiate'op'eration of the associated tone apparatus.'Y Also, the hold relay R380y interrupts,vat

.the contacts .38 I, a normally completed termina- :tioncircuit'alsoincluding the contacts 3| |,'the

resistor 34.4, .the contacts.28|,"lthe condenser 286 and the condensers 341 andl 34E-.across the line conductors C301 and C302 extending '-to the carrierequipment 394. Alsoupon operating the `line relay R3301..comp1etes,;.atrthe' contacts 333,

an obvious path for tapplying ground potential .to lthesignal-:out .conductor C303 extending to y.the carrier equipment1394 in order to effect operation of. thesignal-out relay R391 therein. Upon operating Athesignal-out relay R391in the carrier ...equipment 334.` closes the contacts 398 .included f `answer relay R445 is not poled to operate. this ltime the incorning'selector 429 is conditioned .to be responsive to the first digit .transmitted .thereto over the trunk 435.

brings about. restoration of .the normally operlatter relay. upon restoring closes. the. contacts 496 applying ground potential tothe signal-in conductor C404 in order toeffect operation of thesignal-in relay R450in the :trunk circuit 400. l

Upon-operating the signal-in relay R450 completes, at the contacts 452, an obvious circuit for operating the' hold relayR400; which latter relay upon operating, completes, at the contacts 405, 'a circuit, including the `contacts 485 for energizing the upper winding of the 'switch relay R460 in order to cause. the latter, relay to operate. Upon operating, the switch relay R460 interrupts, at the contacts 466, a normally completed path including the impedance 458 and the contacts 483 vfor applying battery potential to the control lconductor C433 of` `the trunk 430; and completes, at

the contacts 461, a path including the contacts 483, for applying ground potential to the control conductor C433. When battery potential is thus removed from the control conductor C433 and 4ground potential is applied thereto, the trunk 430 is marked as busy to the incoming selectors 434, etc., and totheA selectors of the finderselector links 426, etc., having access thereto; whereby the trunkcircuit 400 is marked as busy to the switches mentioned.

Also, upon operating the switch relay R460 ncompletes, at the contacts 463, a path including winding of the answer relay. R445 across the line .conductors C436 vand C431 of the trunk. 435 ex tending to the incoming selector 429; whereby the the, contactsv 45|, for bridging the right-hand incoming selector 429 operates to return ground potential over the'control conductor C438 of f .the trunk 435 in order to energize in multiple the-left-hand windingof the answer relay R445 and the lower winding of the switch relay R460.

`Accordingly, a holding circuitis thus completed for Aenergizing .the lower winding of the switch relay R460; and the left-hand winding of the At The subscriber at,.the calling subscriber substation T| I in the .remote exchange |0 then dials thev flrstdigit- 1 of the directory.y number of the called subscriber ,substation T2| in the tandem c exchange 20; whereby the line relay R330 in the trunk vcircuit 200 follows the single impulse of the ,iirstdigit lvrestoring and then reoperating. vUpon restoring` andthen lreoperating, the line ...relay R330 interrupts and then recompletes, at

the 'contacts 33|, the previously mentioned circuit for maintaining operated the hold relay R380; whereby the latter'relay being of the vslowto-release type, remains `operated during irnpulsing.

Also, upc'ynfresftc'iring andfthen reopera'tina-the line. relay R330 completes andthenlinterrupts,

at* the contacts 332, .av circuit, including-the contacts 381, for 'Qperating'the dial Vrelay R320V in order to cause thelatter relay to ,operate and ,remainoperated during impulsingas .it ispof .the

by the yaudio busy signal is received by the carrier equipment 394 in the remoteexchange I0,y the balanced demodulator therein demodulates the received signal transmitting the An'iodulating audioV busy signal component thereof overl the line conductors C30| `and C302 that are coupled by way of the previously traced circuit to the line conductors C229 and C221 of the trunk 225. The audio busysignal returned over the line conductors C225 and C221 of the trunk 225 is returned, over the finder-selector link 293 and the subscriber line 2 to the calling subscriber substation T|| inthe remote exchange l0, thereby to' indicate'to thesubscriber thereat that the desired vconnection may not be had at this time by virtuev ofl the busy condition of the called subscribei` substation T2| in the tandem exchange 20.1' The subscriber at the calling subscriber substation TI I in the remote exchange I9 then proceeds to release the apparatus in a manner more fully explained hereinafter.

Now' assuming that the called subscriber substation T2| in' the tandem exchange 20 is idle whenA the local connector 421 operates to select the subscriber line 42| extending thereto, the local connector 421 operates to project ringing current over the subscriber line 42| extending to n the called subscriber substation T2| andto effect operation of the line circuit 423 associated with the subscriber line 42| in order to mark the subscriber line 42| as busy to the other local connectorsin the associated group. Also, the local connector ,'421 returns ring-back vtone kcurrent over the trunk 428, the incoming selector 429 and the line "conductors C436 and C431 of the trunk 435;,V which` audio ring-back tone current effects operationof the carrier equipment 494 in the tandem 'exchange 20 and the carrier equipment 394 inthe remote exchange I0 in the manner previously explained; whereby the audio ringback tone current is returned over the trunk circuit 200, the trunk 225, the finder-selector link 203 and the subscriber line 2| to the calling subscriber substation TII in the remote exchange I0 inV order toindicate to the subscriber thereat that the subscriber at the called subscriber substation T2| in the tandem exchange 20 is being rung.

y,When Athe subscriber at the called subscriber substation T2| in the tandem-exchange 20 answers the call, the local connector 421 operates to reverse the polarityof the trunk 428 and, consequently, the polarity of the line conductors C436 and C431 of the trunk 435 over the incoming ,selector 429; whereby the right-hand winding ofthe answer relay R445 in the trunk circuit 400 is poled to operate. Upon operating the answer relay R445 interrupts, at the contacts 446, a normally completed termination circuit including the condenser 443, the resistor 444 and the'contacts 41| across the line conductors C40| and C402 extending to the carrier equipment 494 and completes, at the contacts 441, an obvious circuit for energizing the upper winding of the supervisory relay R440 in order to cause the latter relay to operate. visory'relay R440 completes, at the contacts 442, an obvious path for applying ground potential to the signal-out conductor C403 in order to eiect operation of the signal-out relay R491 in the carrier equipment 494, Upon operating the signalout relay R491 in the carrier equipment 494 closes the contacts 498 included in the plate circuit of the associated transmitting amplifier; whereby Upon operating, the superthe associated carrier frequency oscillator trans- V mits the 15.95 kc. carrier signal over the line 6,0 extending betweenthe tandem exchange 20 and the remote exchange 10.

When the 15.95 kc.` carrier signal i-s received by the carrier equipment 394 in the remote exchange* I0, the apparatusincorporated therein eifects the restoration of thesignal-in-relay R395 therein. Upon restoring the signal-in relayR395 inthe carrier equipment 394 completes, at the contacts.

396, a path for applying ground potentialjto the signal-in conductor C304 in order to eiect operation of the signal-in relay R350l in the trunk circuit v200. l Upon operating the signal-in relay R350 completes, at the contacts 352,' an obvious circuit' for operating the hold relay R340. Upon operating, the hold relay R340 completes. at the contacts 34|, a circuit including the contacts 385 and 256 for operating shortly thereafter the timer relay R235, the latter relay being of the slow-tooperate type. Upon operating the timer relay R235 completes,- at the contacts 231, a circuit, including the contacts 352, for operating the reverse battery,y relay R240. Upon operating the reverse battery relay R240 reverses, at the contacts 24|, 242,243 and 244, the polarity of the line conductors C226 and kC221 of the trunk 225 with respect, to the upper and lower windings of the line relay R330 in the trunk circuit'200. When l the polarity ofthe trunk 225 is thus reversed, the

meter, not shown, associated with the calling subscriberline 2|| is operated in order to meter the call from the calling subscriber substation TI in the remote exchange I0 to the called subscriber l substation T2| in the tandem exchange20.

. rier equipment394 operates to transmit the lower side band of the 24.55 kc. carrier signal (21.7 to 24.3 kc.) over the line 60. When the carrier equipment 494 in the tandem exchange 20 receives the lower side band of the 24.55 kc. carrier signal (21.7 to 24.3 kc.) it operates to transmit the audio component thereof over the previously traced path including the line conductors C40I and C402 yto-thejcalled subscribersubstation T2|. Similarly, when an audio signal originates at the called subscriber substation T2 it is transmitted over the previously traced path including the line conductors C40|and C402 to the carrier equipment '494 in the tandem exchange 20; whereby the carrierequipment 494 operates to transmit the lower side band of the 15.95 kc. carrier signalr (13.1 to 15.7 kc.) over the line 60. When the carrier equipment 493 in the remote exchange |0 receives the lower side band of the 15.95 kc. carrier signal (13.1 to 15.7 kc.) it operates to transmit the audio component thereof `over the previously traced path including the line conductors'r connects, the local connector 421 again operates 'y to reverse the polarity over the trunk 428 and the incoming selector 429 to the line conductors C435 and Mctsrfv the trunk '435; `whereby um;y o 

